The invention relates to controlling a polyphase electrical machine by means of a plurality of inverters in parallel.
The invention applies more particularly to situations in which such control must present a high degree of reliability, e.g. in the field of aviation.
A known device for controlling a three-phase electrical machine is shown in FIG. 1. The device has two three-phase inverters 1 delivering respective alternating currents (AC) Ia, Ib, and Ic to the phase windings of the electrical machine 4, each inverter supplying about half of each phase current. Each inverter has switches 5 in a bridge circuit, e.g. power transistors. Each inverter has three branches 6, each comprising two power switches 5 connected in series between two opposite terminals +V and −V of a direct current (DC) electrical power supply, and connected to opposite sides of a point 7 that is connected to a respective phase of the electrical machine. A three-pole isolating switch 8 is connected between each inverter and the electrical machine. A circuit 9 controls the switching of the power switches 5 via driver circuits in order to modulate the DC power supply so as to produce alternating currents Ia, Ib, and Ic. Typically, use is made of pulse-width modulation (PWM).
In the event of an inverter branch having a fault, the faulty inverter is isolated by opening the associated isolating switch, and the phase currents must then be delivered in full by the other inverter. Each branch of the other inverter then conveys all of the corresponding phase current, and no longer only half of it. The power switches must therefore be designed to be capable of switching a current having an amplitude that is twice that of the current that they normally convey (when there is no fault).
Another embodiment of a known device for controlling a three-phase electrical machine is shown in FIG. 2. This other embodiment differs from that of FIG. 1 by the presence of an additional inverter with its associated isolating switch. In normal operation, two inverters are in operation, each delivering respective halves of the phase currents, while the third inverter is isolated. In the event of an inverter branch having a fault, the faulty inverter is isolated by opening the associated isolating switch, and the additional inverter is put into operation with its associated isolating switch being closed.
In both of the above configurations, a large amount of overdimensioning is necessary, either in terms of the switching ability of the power switches, or else in terms of the number of inverters.